Clonal strains of rat pituitary tumor cells (GH-cells) are being utilized to study the mechanisms involved in control of hormone secretion. Thyrotropin-releasing hormone (TRH) stimulates prolactin synthesis and secretion by GH-cells. TRH binds to specific membrane receptors, and the concentration of TRH receptors is controlled by TRH, thyroid hormones, estradiol and cortisol. If GH-cells are maintained to culture media containing sera from a thyroidectomized animal, addition of physiological concentrations of T3 or T4 leads to a loss of 50% of TRH receptors. This receptor loss occurs at the same time and at the same T3 concentrations as loss of cellular responsiveness to TRH as measured by prolactin synthesis. The possible role of nuclear T3 receptors in this process will be investigated by comparing T3 binding, TRH receptor concentrations, and another response to T3, stimulation of growth hormone synthesis. Other hormones which modulate TRH receptors include estrogen and cortisol. The synergistic effects of T3 and these steroid hormones is under investigation. A mouse TSH-producing tumor will be maintained in either primary or permanent culture. These cells will be used to determine whether thyroid hormones block TRH actions and regulate TRH receptors in thyrotrophs as well as in mammotrophs. Synchronous cultures of GH-cells are being obtained by physical detachment of mitotic cells. This cell cycle dependence of prolactin and growth hormone synthesis and TRH responses will be measured. Immunocytochemical methods will be utilized to determine the fraction of cells containing growth hormone and prolactin in G1, S, G2 and M.